Insulin immunoactivity and bioactivity are detected in whole chick embryos before pancreatic beta cell differentiation. Since insulin is apparently a requirement for normal development, we are investigating the pattern of expression of the insulin gene in the developing chicken pancreas and in the whole embryo at pre- pancreatic stages. Insulin-like growth factor binding also is being studied in chick embryo tissue. Using labeled insulin-like growth factor I and II (multiplication stimulating activity) and unlabeled homologous and heterologous peptides in competion binding assays, it appears that there are a specific insulin-like growth factor I receptor but not insulin-like growth factor II receptor in the developing chick embryo. The growth factor binding pattern is different from insulin binding in several chick embryo tissues. To define insulin's role in early development anti insulin antibodies were injected into fertilized eggs and the effect of antibodies was studied. Insulin receptor and insulin-like growth factors (I and II) receptors structure studies have been extended in rat brain. The binding of labeled peptides to thin sections of frozen fresh rat brain was visualized with autoradiography. By several criteria including structure-activity relationship analysis, these brain peptide receptors were qualitatively indistinguishable from peptide receptors previously characterized on brain and other more typical target tissues and distinct from each other. Each peptide exhibits its own distinctive binding pattern - i.e., each peptide binds to cytoarchitectonic structures.